Molecular devices based on metallocyclam subunits

Encircling of a transition metal by the 14-membered satd. tetra-aza macrocycle cyclam provides some useful properties which are not obsd. for corresponding complexes with open-chain counterparts: high thermodn. stability, extreme inertness towards demetallation, enhanced redox activity. Thus, a variety of functionalities can be appended to the cyclam framework to generate systems capable of developing several activities. As an example, covalent linking of a fluorogenic fragment to cyclam gives a system whose light emission can be controlled (switched on/off) by changing the oxidn. state of the encircled metal (either nickel or copper). Moreover, a pendant arm can be appended to the cyclam framework, displaying either an acid-base or a redox activity. As a consequence, binding of the pendant arm to the metal center can be controlled by varying the pH and the redox potential, resp., thus generating a defined mol. motion. Finally, the quite strong in-plane interaction exerted by cyclam on the nickel(II) ion gives rise to a temp. controlled high-spin/low-spin interconversion, which may influence the light emission of a linked fluorophore: this produces a sensitive fluorescent mol. thermometer.